1. Field of the Invention
The invention relates to a burst mode LLC resonant power converter and, in particular, to a burst mode resonant power converter with high conversion efficiency.
2. Description of Related Art
As shown in FIG. 7, a burst mode LLC resonant power converter in the prior art includes a rectifier 70, a power factor correction (PFC) circuit 71, a resonant circuit 72, a controller 73, and a burst mode triggering unit 74.
The rectifier 70 is connected to an AC power supply to convert AC power to a sinusoidal DC power for output.
The PFC circuit 71 is connected to the rectifier 70 to detect the voltage and current of the sinusoidal DC power and to adjust the PFC thereof, thereby outputting a DC power. The PFC circuit 71 includes an energy-storing inductor L1, a power switch S1, and an energy-storing capacitor Cbulk. One end of the energy-storing inductor L1 is connected to the output end of the rectifier 70, and the other end is connected to the power switch S1 and the energy-storing inductor L1.
The resonant circuit 72 includes a transformer T1, a resonant unit Lr and Cr, a half bridge switch circuit 721, and an output capacitor COUT. The resonant unit Lr could be the leakage inductor of the primary coil of the transformer T1 or an independent inductor. The transformer T1 has a primary coil and a center-tapped secondary coil. The resonant unit Lr and Cr is connected between the bridge switch circuit 721 and the primary coil. The output capacitor COUT is connected to the center-tapped secondary coil. The resonant unit Lr, Cr is connected to the bridge switch circuit 721. The resonant unit Lr, Cr of the resonant circuit 72 is an LC circuit. Since the LC circuit Lr, Cr is connected to the primary coil of the transformer T1, it forms a resonant circuit with a magnetizing inductor (not shown) of the primary coil. The resonant circuit 72 has two resonant frequencies. The first resonant frequency is given by the magnetizing inductor of the primary coil and the resonant capacitor Cr of the LC circuit. The second resonant frequency is given by the magnetizing inductor (not shown), leakage inductance and the resonant capacitor Cr.
The controller 73 includes a reference voltage input end PFCSV, multiple output ends PFCG, GHS, HLS, an output voltage feedback end FB, an maximum switch frequency setting end RFMAX, and a burst mode triggering end SNOUT. The reference voltage input end PFCSV is connected to the energy-storing capacitor Cbulk via a voltage divider R1, R2. The output ends PFCG, GHS, HLS are connected to the power switch S1 of the PFC 71 and the bridge switch circuit 721 of the resonant circuit. The output voltage feedback end FB is connected to the filter capacitor COUT of the resonant circuit 72, i.e., the output end of the power converter, via a photo coupler 731. The maximum switch frequency setting end is connected with a fixed resistor R10 in order to determine the maximum switch frequency.
The burst mode triggering unit 74 is connected to the burst mode triggering end SNOUT of the controller 73 and the photo coupler 731 to detect the voltage output by the power converter. The burst mode triggering unit 74 determines whether the load status is in a no-load condition or a light-load condition and sends determined results to the burst mode triggering end SNOUT. The burst mode triggering unit 74 comprises a comparator 741 and an electronic switch Sb. An inverting input end of the comparator 741 is connected to the photo coupler 731, and a non-inverting input end is connected to a fixed reference voltage. An output end of the comparator 741 is connected to the electronic switch Sb to determine its on and off. The electronic switch Sb is connected to the burst mode triggering end SNOUT of the controller 73.
The controller 73 outputs a burst width signal whose duty cycle is 50% to the bridge switch circuit 721, making the high and low active switches HS, LS of the bridge switch circuit 721 become conductive alternately. In this case, the power converter outputs a stable DC voltage. When the load is in the no-load condition or the light-load condition, the output voltage of the power converter increases. The electrical current through the photo coupler 731 rises, pulling down the potential of the output voltage feedback end FB. Therefore, the controller 73 performs switch controls on the half-bridge switch circuit according to the maximum switch frequency determined by the fixed resistor R10 connected with the maximum switch frequency setting end RFMAX. With reference to FIG. 9, when operating at the maximum switch frequency, the overall gain decreases and, therefore, the output voltage drops. If the output voltage still remains in the no-load condition or the light-load condition, the electrical current may be too large. The potential on the output voltage feedback end FB is still low. In this case, the comparator 741 outputs a high potential signal to the electronic switch Sb. As a result, the electronic switch Sb becomes conductive and pulls down the potential on the burst mode triggering end SNOUT, triggering the burst mode of the controller 73. With reference to FIG. 8 two sets of 50% burst width signal waveforms are output from the controller 73 to the bridge switch circuit 721 under the burst mode. After the controller 73 enters the burst mode, each 50% burst width signal will be inserted with several blank cycles, further dropping the output voltage to maintain a stable voltage output.
According to the above description, even though a lower gain is obtained by operating below the maximum switch frequency to suppress the rise in the output voltage under the no-load condition or the light-load condition, the switching loss increases. The switching loss inevitably lowers the efficiency of the power converter at light or no load. This may not satisfy the international standard for no-load or light-load conditions. It is thus imperative to provide a better solution.
To overcome the shortcomings, the present invention provides a burst mode resonant power converter with high conversion efficiency to mitigate or obviate the aforementioned problems.